British pipe sizes are designated BSP.Tr for tapered threads, and BSP.Pl for parallel threads. Pipe sizes are the same both sides of the pond, and there is no 7/8 pipe.
There is however a BSP.F thread that is similar to our NS(National Special) series of thread. The BSP that is being referred to for the spindle threads is the BSP.F, 7/8-14 which has a O.D. of 1.189in. The BSP.F series of thread uses the same nominal diameters as the British pipe schedule, but have added a few sizes in between the standard pipe sizes to give a better range of thread sizes.
Perry
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